Electrical angular displacement sensor

ABSTRACT

PCT No. PCT/GB85/00387 Sec. 371 Date Dec. 1, 1986 Sec. 102(e) Date Dec. 1, 1986 PCT Filed Aug. 30, 1985 PCT Pub. No. WO86/01588 PCT Pub. Date Mar. 13, 1986.The sensor has at least one strain gauge secured to a strip sufficiently flexible to bend without permanent plastic deformation through a small radius. The output is directly proportional to the relative angular orientation of the ends of the strip regardless of the orientation intermediate the ends.

This invention relates to an electrical sensor for measuring angulardisplacement. Particularly, but not exclusively, the invention relatesto an electro-gonio-meter for use in monitoring joint mobility inpatients with ailments such as arthritis which produce abnormal jointmovement.

One electro-goniometer for use in medical assessment of joint mobilityuses mercury-filled tubes as resistive elements of a Wheatstone bridgecircuit. Two or four such tubes are used to provide a bendable beam anda linear relationship between electrical output and angular displacementis given. Problems have been encountered concerning the long-termstability of the electrode/mercury interface and also thewell-recognised toxicity of mercury makes its use undesirable.

Another known type of electro-goniometer for orthopaedic diagnosis usesan electrically conductive impregnated plastics strip. Although thisarrangement allows a small flexible unit to be made great difficultieswith the calibration and electrical stability have been encountered.

An object of this invention is to obviate or mitigate the aforesaiddisadvantages.

According to the present invention there is provided an electricalsensor for use in providing a signal indicative of angular displacementbetween two spaced locations, comprising a flexible carrier of materialcapable of flexing without permanent plastic deformation when its endsare fixed to said locations and said locations undergo relative rotarymovement within a predetermined range; and at least one electricallyconductive element having a resistance which is a function of strainaffixed to the carrier and extending therealong substantially the wholedistance between said locations.

Preferably the carrier is a thin strip of spring steel not exceeding 0.5mm in thickness. Other possible materials are, for example, non-ferrousmetals, acrylic polymers, thermoplastics materials and other plasticsmaterials and fibres.

The strain may be monitored by measuring the electrical resistance ofthe gauges in a Wheatstone bridge arrangement.

The electrical output may be calibrated to read angular displacementbetween the ends of the carrier.

The criteria employed in producing the electro-goniometer of thisinvention were to produce a device which:

(a) required no special alignment with the rotation axis of the limbs,

(b) had low stiffness to facilitate fixation to limbs,

(c) was small enough to be worn beneath a patient's clothing,

(d) has long term electrical stability, and

(e) has infinitely variable electrical output.

The electro-goniometer of the invention uses the `bending beam` approachbut replaces the mercury filled tubes of prior devices, suitably withcommercially available strain gauges. These gauges have a definitestrain limit and have, up till now, been used to measure strain, stressor force where no appreciable angular displacements have occurred. Theuse of strain gauges where significant deflections would take place is arather controversial step but the strain magnitude has been reducedsufficiently by decreasing the thickness of the bending beam to verysmall values.

The resulting device is therefore very flexible and tests have shown theelectrical output to be linearly related to angular movement andstability to be excellent. In addition, the method of bending does notinfluence the output (a Z bend would zero if the top and bottom ends ofthe carrier were parallel). The size of the unit can be very small andwill therefore easily satisfy the criterion of being worn beneathclothing. The device therefore oversomes several of the limitations ofprevious devices and is convenient for the measuring of joint motion inclinical and research centres.

The mechanics involved in the design of the bending beam originate fromthe equation for pure bending: ##EQU1## where σ=stress in a particularfibre (usually surface)

Y=distance from neutral axis of beam to the fibre in question

M=Bending moment applied to beam

I=Moment of intertia of beam (geometry)

E=Young's Mondulus of Elasticity for material

R=Radius of curvature of deformed beam

For the use of a bending beam as an electro-goniometer it is preferableto have zero bending moment (or resistance to bending movement)therefore the equation: ##EQU2## is used giving ##EQU3##

Since the strain gauges measure the strain (change in length divided bythe original length) is it necessary to relate stress (σ) to strain (ε)using Youngs Modulus:

    σ=εE

The bending equation therefore reduces to ##EQU4##

In other words the surface strain of a bending beam is directly relatedto the thickness (2y) and inversely related to the radius of curvatureof the deflected beam. For normal strain gauges the limit of strain willbe in the order of a few thousand microstrain (i.e. 1000-10,000×10⁻⁶).An example of the scale of strain means that a beam 5 mm thick cannotbend tighter than a 250 mm radius arc without the danger of plasticdeformation. ##EQU5##

For normal operating conditions a working strain of 1000×10⁻⁶ is idealwhich would result in a radius of curvature of 2.5 m for a 5 mm thickbeam.

For use as a flexible electro-goniometer for joint assessment thebending beam must be able to conform to a much smaller radius ofcurvature. Values as low as 5 mm are required in some operatingconditions which means the thickness of the beam must be recalculated.##EQU6## This value is extremely small and for practical limits ofthickness of shim to 0.05 mm results in larger strains or larger radiiof curvature. Alternatively a bend of 50 mm radius would suit a 0.05 mmshim.

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic perspective view of a device embodying theinvention;

FIG. 2 is a perspective view showing a device according to the inventionapplied to a hinged joint;

FIG. 3 shows a circuit incorporating the device;

FIG. 4 is a typical calibration graph;

FIG. 5 is a schematic perspective view of an alternative embodiment formeasuring shear strain or twisting displacement; and

FIG. 6 illustrates a device combining the features of FIGS. 1 and 5.

Referring to FIG. 1, an electro-goniometer 1 comprises an elongatecarrier strip 2 of spring steel having an elongate strain gauge 3adhesively secured thereto and aligned with the longitudinal axis of thestrip 2. The strain gauge 3 extends along substantially the whole lengthof the carrier strip 2. Any commercially available strain gauge ofsuitable dimensions may be used; examples are strain gauges sold asKyowa KC-120-Al-11 and Showa N.11-FA-60-120-11. The strip 2 in thisembodiment has the following features:

    ______________________________________                                        Length             125 mm                                                     Width              4 mm                                                       Thickness          0.05 mm                                                    Material           High carbon steel                                          ______________________________________                                    

A second, similar strain gauge is preferably provided on the opposedface of the strip 2.

FIG. 2 shows the device 1 applied to a hinged joint 4 capable ofrelative rotation to give a variable angle θ. The joint 4 is shown forsimplicity as a pair of plates hinged together, but in practice could befor example an ankle joint. The device 1 is secured at its ends by anysuitable means, such as adhesive tape 5. On any convex curve of thestrip 2 the strain gauge 3 is stretched and thus its resistance rises,while any concave curvature produces a reduction in resistance. The netchange in resistance is thus a function of the angle θ, regardless ofthe curvature or number of curves intermediate the ends of thegoniometer 1.

FIG. 3 illustrates one circuit suitable for use with the presentinvention. A goniometer 1 having series front and rear strain gauges 3,3a is connected in a bridge circuit with resistors 6, 7. Furtherresistors and/or amplifiers may be included in the bridge to improvesensitivity. The resistors 6, 7 may be fixed resistors in a monitoringcircuit, or alternatively may comprise further strain gauges adjacentthe goniometer to provide compensation for ambient conditions such astemperature. It is also possible to provide all four arms of the bridgeas strain gauges on a single carrier.

As seen in FIG. 4 the voltage measured across the goniometer of theinvention is linearly proportional to the angle θ between its ends. Thusa meter such as 8 (FIG. 3) can readily be calibrated to give a readingdirectly in angular measurement.

Turning to FIG. 5, the invention also provides a sensor 10 formonitoring twisting movement. A carrier strip 11, similar to that of theprevious embodiment, has secured thereto a pair of sepentine straingauges 12, 13 comprising lengths arranged oblique to the longitudinalaxis of the strip 11; preferably the mean angle is 45°. The sensor 10acts in a manner analogous to the above embodiment to give an outputindicative of the relative rotational twist between its ends about thelongitudinal axis.

The embodiments of FIGS. 1 and 5 can be combined as shown in FIG. 6.Such a device provides means for defining the three-dimensional angularorientation of one end of the device relative to the other.

I claim:
 1. An electrical sensor for use in providing a signalindicative of angular displacement between two spaced locations,comprising a generally planar flexible carrier of material capable offlexing when its ends are fixed to said locations and said locationsundergo relative rotary movement within a predetermined angular range;and at least one electrically conductive element having a resistancewhich is a function of strain affixed to the carrier and extendingtherealong substantially the whole distance between said locations;wherein said electrically conductive element comprises a wire resistancestrain gauge and the thickness of said carrier is selected such that thestrain at the surface thereof does not exceed a predetermined limit whenthe carrier is flexed by relative rotary movement of said locationswithin said predetermined angular range, said limit being selected suchthat there is no permanent plastic deformation of the carrier or thestrain gauge when the relative movement of said locations within saidpredetermined angular range causes maximum flexure of the carrier; andwherein the thickness of the carrier is determined according to theequation:

    Y=εR;

where Y is half the thickness of the carrier, ε is the maximumpermissible strain at the surface thereof, and R is the minimum requiredradius of curvature of the carrier.
 2. The device of claim 1 in whichsaid element comprises one or more conductor tracks located on one faceof the carrier and aligned with the longitudinal axis of the carrier. 3.The device of claim 2 including a second element of like form affixed toan opposite face of the carrier.
 4. The device of claim 1 comprising twoelements affixed to one face of the carrier, each element comprising aserpentine path of sections oblique to the longitudinal axis of thecarrier, the oblique orientations of the two elements to said axis beingsubstantially equal and opposite.
 5. The device of claim 4 including athird element located on the same face of the carrier as said twoelements and comprising one or more conductor tracks aligned with thelongitudinal axis of the carrier.
 6. The device of claim 5 including afourth element of like form to said third element and affixed to theopposite face of said carrier.
 7. The device of claim 1, 2, 3, 4, 5, or6 wherein the maximum permissible strain is 1000 microstrain.
 8. Thedevice according to any of claims 1, 2, 3, 4, 5, or 6 in which thecarrier is capable of flexing without permanent plastic deformationthrough a radius of curvature of less than 250 mm.
 9. The deviceaccording to any of claims 1, 2, 3, 4, 5, or 6 in which the carrier is aspring steel strip of thickness 0.5 mm or less.